The present invention relates to a method of controlling the function of a gas-separating centrifugal pump and vacuum pump combination, and a gas-separating centrifugal pump. The apparatus according to the invention is particularly well suited for use as a fluidizing centrifugal pump for pumping pulp of medium consistency (e.g. about 6-15%), but the method and the centrifugal pump utilizing it can also be used in other applications in which the liquid to be pumped contains gas and solid matter.
Earlier known pumps which are used for the above purpose are described, inter alia, in U.S. Pat. Nos. 4,776,758, 4,981,413, 5,078,573, 5,114,310, 5,116,198, 5,151,010, 5,152,663 and 5,366,347. All of the above-mentioned patents deal primarily with pumps for the wood processing industry, which separate gas from pulp suspensions of medium consistency and which are characterized in that, in addition to the conventional impeller, a vacuum pump, preferably a water ring pump, is mounted on the pump shaft in a chamber behind the impeller. Gas outlet openings, through which gas accumulating in front of the impeller of the centrifugal pump can flow to the volume behind the impeller, are disposed in the back plate of the pump impeller, near the impeller shaft. This volume is, in most cases, connected to the suction opening of the vacuum pump through a gas outlet duct at least partly surrounding the pump shaft. When the vacuum pump creates a pressure difference between the volume in front of the impeller and its own pumping chamber, the gas flows through the openings in the impeller and the gas outlet duct at least partly surrounding the shaft to the chamber of the vacuum pump. Because of the eccentricity of its chamber, the vacuum pump creates, in a manner known per se, suction which draws gas into its chamber, and a pressure difference between the atmosphere and its chamber on its outlet side so that the gas is discharged from the chamber of the pump. Usually the separated gas is discharged from the vacuum pump directly to the atmosphere.
Certain special requirements are applied to the centrifugal pump and vacuum pump combinations used for pumping pulp suspensions of the wood processing industry, which have been extensively dealt with in the above-mentioned patents and can therefore here be dealt with relatively briefly.
Firstly, since the material to be pumped contains solid matter, i.e. cellulose fibers, provisions have to made in the construction of the centrifugal pump and the vacuum pump connected to it for the possibility that fibers get into the gas outlet system. For that reason, the back side of the back plate is, for example, provided with back blades, in order to separate fibers from the material which has found its way to the volume behind the impeller. As fibers can also get into the vacuum pump, flushing means are provided both on the suction side and the outlet side of the pump in order to prevent clogging of the ducts by fibers.
Secondly, the conditions can vary considerably when pumping fiber suspensions. The consistency of the pulp, for instance, can vary by several percentage points and the inlet pressure of the pulp by several bars. Since gas removal in front of the impeller, in order to function reliably, requires a certain pressure difference, the inlet pressure must be taken into consideration such as by providing control of the suction of the vacuum pump. This is usually accomplished by providing an auxiliary air duct connected to the suction duct and through which extra air can flow to the vacuum pump when enough gas is not separated in front of the impeller. A valve which opens at a given pressure, e.g. 0.4 bar gauge, is usually connected to the auxiliary air duct.
Thirdly, when pumping fiber suspensions the separated gas does not in most cases consist of pure air, but may often contain various malodorous or even to some degree poisonous or corrosive gases, which should not be discharged directly to the atmosphere. Fibers also get into the outlet of the vacuum pump to some extent, and it should be possible to recover them, so that the outlet pipe of the vacuum pump should not, for that reason also, be connected directly to atmosphere or a drain.
Attempts have been made to fulfil the first two of the above mentioned basic requirements, such as in U.S. Pat. No. 5,366,347, which is based on the idea that a fluidizing centrifugal pump pumping pulp of medium consistency has to be able to operate under three different operating conditions.
In the first case, where the inlet pressure is low, below the atmospheric pressure, a great amount of gas is separated in front of the impeller, so that the capacity of the vacuum pump must be high and the pump has to be able to remove all the gas separated.
In the second case, where the inlet pressure is medium, only slightly above the atmospheric pressure, gas is separated in front of the impeller to some degree, and it must be possible to remove it through the vacuum pump without entraining fibers.
In the third case, where the inlet pressure is high, for instance above 2 bar gauge, no gas is separated and the vacuum pump has nothing to remove.
The '347 patent suggests that the capacity of the vacuum pump should be controlled by moving the housing of the vacuum pump in relation to the rotor of the vacuum pump. The idea is that the vacuum pump in the first operating condition sucks gas from the vacuum space in front of the impeller and is capable of transporting it to a higher, i.e. atmospheric pressure. The pump functions in this case as it is originally meant to function.
In the second operating condition where the gas pressure of the separated gas is above the atmospheric pressure, the housing of the vacuum pump is moved in relation to the rotor into a position in which the vacuum pump creates a pressure difference in opposite direction to that of the first case. In other words, assuming that the inlet pressure of the pulp causes an absolute pressure of 1.5 bar in front of the impeller, the pressure difference in relation to the atmosphere is 0.5 bar. As the pressure difference is relatively great, a counter pressure of for instance 0.3 bar overpressure is produced by means of the vacuum pump, so that the pressure in front of the impeller first has to surpass the counter pressure of the vacuum pump. The gas will in other words flow out to the atmosphere at a pressure difference of only 0.2 bar.
For the third operating condition, the '347 patent suggests that the eccentric housing of the vacuum pump be moved so as to be concentric with the shaft and the rotor of the vacuum pump. That is the pump does not generate any pressure difference in either direction. Presumably, it is assumed that since no gas is separated in front of the impeller, no fibers are able to pass into the gas outlet, in spite of the great pressure difference. However this misses a significant point: when a considerable overpressure exists on the suction side of the centrifugal pump, it tends to cause fluent material to burst out from the pump through all available passages. If the vacuum pump, as described in the '347 patent, is running "idle", i.e. the housing of the vacuum pump is concentric with the rotor and no valve is disposed on the outlet side of the vacuum pump, the absence of which is stated to be an advantage, the pulp suspension (under overpressure) will obviously flow directly through the vacuum pump along the gas outlet channels.
The above mentioned problem could be solved in the pump according to the '347 patent in at least two ways: by arranging a valve on the outlet side of the vacuum pump so that the valve would be closed or throttled when the pump is running "idle", and consequently the whole gas outlet pipe system would be at least partly closed; or by improving the capability of the vacuum pump to produce counter pressure so that the maximal counter pressure generated by the pump would correspond to the highest possible overpressure on the suction side of the centrifugal pump. It has thus on one hand been suggested in the '347 patent that in case of a slight overpressure on the suction side of the centrifugal pump, the eccentricity of the housing of the vacuum pump should be changed so that the vacuum pump produces a counter pressure great enough to "dampen" the overpressure. On the other hand, it is also suggested that the eccentricity of the housing of the vacuum pump be further decreased to zero when the overpressure on the suction side of the centrifugal pump increases. However the latter suggestion results, in practice, in a pump that leaks excessively. The matter can however easily be corrected by increasing the eccentricity of the housing of the vacuum pump as well, so that the counter pressure produced by the vacuum pump increases when the overpressure of the centrifugal pump increases. In other words, by keeping the counter pressure produced by the vacuum pump the same as the inlet pressure, there will be no flow in either direction in the vacuum pump. The effect of the inlet pressure can naturally be reduced also by providing a throttling valve on the outlet side of the vacuum pump, contrary to the teaching of the '347 patent, so that the inlet pressure can be "dampened" by means of the throttling valve as well as by changing the eccentricity of the housing of the vacuum pump. In other words, the arrangement described in the '347 patent can be corrected simply by providing a sufficient margin for the eccentricity adjustments considered to be required for the housing. All of the features described in the '347 patent can be utilized and the disclosure of U.S. Pat. No. 5,366,347 is incorporated by reference herein.
The pump described in more detail in the '347 patent does not, even after the above mentioned corrections, wholly correspond to the requirements which are currently typically applied to pumps in pulp mills, including because the gas to be removed often can contain malodorous or poisonous chemicals. Also a small amount of liquid, a few liters per minute, and in some cases also fibers continually discharge from the vacuum pump. As it would be advantageous from an environmental point of view as well as considering recovery of fibers and chemicals to conduct the exhaust from the vacuum pump to a separate location instead of a "drain", the design of a centrifugal pump and a vacuum pump should consider that the vacuum pump should be capable of discharging the gas, fibers and liquid to a pressured volume, or at least to a location above the pump. The pump must, in other words, besides being capable of generating a vacuum on its suction side, also be capable of producing a head or overpressure on its outlet side.
In the above mentioned patents, the desirability of producing both a vacuum and overpressure has either not been effectively taken into consideration or has not, for other reasons, been dealt with at all. In most of the patents, the control of the pump combination has not been dealt with in any way. In some patents, it has been suggested that a stop valve can be provided on the outlet side of the vacuum pump, by means of which the outlet can be throttled or, if required, even closed. This functions well until the valve actually has to be fully closed. When closed, the valve causes cavitation and pressure shocks in the vacuum pump, greatly increasing the risk that the vacuum pump will be damaged. Another possibility is to change the capacity of the pump, as described in the '347 patent. Controlling the capacity, however, means that the pump no longer has the head required to transport the gas and/or fibers and/or liquid forward. This can be explained by the following example. In the case where only a small amount of gas is separated and only a small vacuum is needed for removing the gas from the centrifugal pump, the vacuum pump is adjusted to generate only a small pressure difference. From this follows that correspondingly only a small pressure difference is available on the outlet side of the pump, which is not enough if, for instance, the exhaust of the pump must flow to a location about twenty meters higher, and sometimes even slightly pressurized.
The above problem has been solved by the method and apparatus according to our invention by providing a control means on the suction side of the vacuum pump, so that the vacuum generated by the vacuum pump in front of the impeller of the centrifugal pump can be controlled totally regardless of the capacity of the vacuum pump. In other words, although only a small vacuum effect is directed towards the centrifugal pump side, the whole capacity of the vacuum pump is available for removal of separated gas, fibers and liquid.
According to one aspect of the present invention a method of operating a centrifugal and vacuum pump combination in which the centrifugal pump has an impeller disposed on the same shaft as a rotor of the vacuum pump, and a gas outlet duct extends between the centrifugal and the vacuum pump, is provided. The method comprises the steps of: (a) operating the pumps so that as the centrifugal pump pumps fluent material, gas is separated from the material, and the vacuum pump draws the gas from the centrifugal pump through the gas outlet duct; and (b) positively controlling the flow of the gas passing through the gas outlet duct between the centrifugal pump and the vacuum pump. The gas outlet duct typically has a given cross-sectional flow area and step (b) may be practiced by changing the effective cross-sectional flow area, e.g. by providing a flexible tubular element in a groove formed adjacent the outlet duct by controlling the flow of fluid to the flexible tubular element to cause it to expand and contract thereby control the cross-sectional area of the gas outlet duct.
The vacuum pump may include a suction opening having a predetermined cross-sectional area and positioned in the gas outlet duct, and step (b) may be practiced by changing the cross-sectional area of the vacuum pump suction opening.
The fluent material pumped by the centrifugal pump is preferably a slurry, e.g. a cellulose fiber slurry having a solids consistency of between about 6-15%. Step (b) may be practiced automatically in response to a conventional consistency sensor sensing the solids consistency of the slurry being pumped by the centrifugal pump. Step (b) may alternatively or also be practiced automatically in response to the sensing (utilizing a conventional pressure sensor) the inlet pressure to the centrifugal pump of the slurry being pumped. Alternatively step (b) may be practiced automatically in response to the gas content of the material being pumped.
The method may also include the further step of discharging gas from the vacuum pump at superatmospheric pressure to a confined volume that is also at superatmospheric pressure. The vacuum pump rotor may be spaced from a housing wall of the vacuum pump, and step (b) may be practiced by changing the spacing between the rotor and the housing wall, as by rotating the vacuum pump housing which preferably is eccentric.
According to another aspect of the present invention a pump is provided comprising the following components: A volute casing and a pump body. A centrifugal pump impeller mounted for rotation by a shaft in the volute casing. A suction opening in the volute casing, and a substantially tangential outlet extending from the volute casing. The impeller including a back plate having a front side facing the suction opening and an opposite back side. At least one working blade connected to the first side of the back plate, and at least one back blade connected to the second side thereof. The pump body including a vacuum pump having a housing and containing a rotor with rotor blades, the rotor mounted on the shaft. The vacuum pump housing including a rear wall and a front wall, the front wall adjacent the volute casing and the rear wall spaced from the front wall and the volute casing, the front wall having a suction opening therein. The vacuum pump housing further comprising an eccentric inner wall surrounding the rotor, an auxiliary air channel, and an outlet leading from the vacuum pump housing to the exterior thereof. A back wall of the volute casing disposed between the impeller back plate and the vacuum pump housing front wall. A gas outlet duct extending through the back wall from the volute casing and the suction opening. And, a control device disposed in the gas outlet duct for controlling the flow of gas through the outlet duct.
Various distinctive features may be provided in the pump as described above. These distinctive features may include the following: The outlet duct is defined by a wall and the control device comprises at least one plate moving in a groove disposed in the outlet duct wall. The at least one plate is movable in either the axial, radial, or peripheral dimension with respect to the shaft. The control device comprises an element disposed in the outlet duct and expandable in the axial, radial, or both axial and radial directions with respect to the shaft, to thereby control the effective cross-sectional area of the outlet duct. The control device element comprises a tube of flexible material and a fluid for expanding or contracting the tube provided therein. The suction opening is disposed in a rotatable element and the control device is operated by rotating the rotatable element. The rotatable member comprises the vacuum pump housing front wall. The control member comprises a ring mounted for movement in the axial direction with respect to the shaft, the ring defining, with the impeller, the gas outlet duct. The ring is movable in the axial direction by expansion or contraction of a fluid filled tubular member and a spring. The gas outlet duct includes an expansion chamber. The auxiliary air duct leads to the expansion chamber. A fluidizing roller is provided which protrudes from the impeller on an opposite side of the impeller from the vacuum pump housing. The vacuum pump outlet leads from the vacuum pump housing rear wall. The gas outlet duct is defined by a spacing between the back wall of the volute casing and the front wall of the vacuum pump housing.
According to another aspect of the present invention the pump, as described above, may have the vacuum pump housing movable into a first position when the inlet pressure to the centrifugal pump is low and a high volume of gas is separated; a second position when the inlet pressure is slightly above atmospheric and a lesser volume of gas is separated; and a third position when the inlet pressure is superatmospheric, the eccentricity of the vacuum pump housing being greater in the third position than in the second position. A throttling valve may be disposed in or adjacent the outlet from the vacuum pump housing.
The invention also relates to a method of controlling a pump as described above by primarily controlling the pressure difference across the vacuum pump by changing the eccentricity of the vacuum pump housing during all operating conditions. The controlling step is practiced by automatically moving the vacuum pump housing between the first, second, and third positions in response to the sensing of the inlet pressure.
It is the primary object of the present invention to provide a simple yet advantageous method of operating a centrifugal and vacuum pump combination, and pump per se. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.